The present invention relates to a ceramic electronic part of integrally sintered structure for use in microwave circuits, etc.
Multilayered electronic parts are small-sized electronic parts widely used in a frequency range from low frequency to microwave, and produced by laminating a plurality of ceramic green sheets having thereon printed electrode patterns and integrally sintering the laminated green sheets. The multilayered electronic part includes a single-functional part such as a multilayered chip capacitor, a multilayered inductor and a multilayered transformer, and a multifunctional part such as a band pass filter (BPF), a low pass filter (LPF), a high pass filter (HPF), an antenna switch, a coupler, etc.
As the ceramic material, various dielectric materials and magnetic materials have been used depending on the required performance of the multilayered electronic part. Typical dielectric material may include barium titanate, calcium titanate, calcium zirconate, lead titanate, lead titanate zirconate, alumina, etc. The magnetic material is typically a soft ferrite such as NiZn ferrite, etc.
The material for the electrode pattern must retains a good conductivity even after the sintering process, and Ag and Ag-based alloys which are relatively difficult to be oxidized at a high temperature have been used. The sintering temperature of laminated ceramic sheets is usually at 1100.degree. C. or lower in view of the melting point of Ag and Ag-based alloys. To ensure the low sintering temperature, a low-melting oxide constituting a grain boundary phase of the ceramic is used as a secondary component together with the ceramic material constituting a main phase as a primary component.
However, when a ceramic sheet containing the low-melting oxide and having thereon electrode patterns as the internal electrode made of Ag or Ag-based alloys is sintered in air, the ceramic sheet is blackened and the dielectric loss of the ceramic is unfavorably increased particularly in the microwave region. Due to these problems, a known electronic part having an internal electrode made of Ag or Ag-based alloy has been inferior in appearance and/or performance, and difficult to be commercialized without an additional specific treatment.
There has been another problem in the known electronic parts.
Usually, after sintering laminated ceramic green sheets, the sintered product is made into a final electronic part through several steps of baking thereon external electrodes, plating the external electrodes with Ni, solder, etc. and printing markers for identifying or distinguishing product number, etc. Error in the printed markers, even only one character or only one symbol, will cause a misuse of the parts to result in a failure in obtaining the intended performance of the electronic parts or result in a serious accident. For example, if a position marker for an input terminal is printed on the output terminal side, a mismounted part will fail to exhibit the intended function. In addition to indicating the kind, etc. of the final products, the markers printed during the production process serve as indicators for controlling the position of semi-fabricated products in the automatic production system. Further, the markers are used in controlling or checking the product quality, etc.
The markers have been provided at various stages of the production process at a time or several times according to the intended use, for example, at the final stage of production, before sintering the green sheets, etc. Before completing the production, the integrally sintered body is exposed to severe conditions. For example, it is dipped in a strongly acidic plating solution, receives various physical or mechanical forces, etc. Therefore, the markers provided to a semi-fabricated product are likely to be corroded, disappear, become unclear, etc. when exposed to severe conditions. This increases the cost of selecting and resorting the products, thereby increasing the production cost of the electronic parts. In particular, the markers provided before the plating process have been practically impossible to be used for identifying or distinguishing the final products. In addition, when the electrode printed on a dielectric ceramic is made of a material containing Ag, the blackening of the ceramic body makes the markers difficult to be visually distinguished.
Japanese Patent Laid-Open No. 8-259263 discloses a glass ceramic dielectric material having a high relative dielectric constant and a low dielectric loss at microwave region. It is taught that since the glass ceramic dielectric material can be sintered at a low temperature, silver and copper can be used as the material for electrodes and conductors. The glass ceramic dielectric material comprises 40-90 weight % of a glass powder and 60-10 weight % of a ceramic powder, the glass powder comprising 10-35 weight % of SiO.sub.2, 5-35 weight % of Ln.sub.2 O.sub.3 (oxides of lanthanoid), 15-50 weight % of TiO.sub.2, 3-45 weight % of RO (oxides of alkaline earth metal), 1-30 weight % of Bi.sub.2 O.sub.3 and 0-25 weight % of ZrO.sub.2. The glass powder may be added with PbO up to 30%. The ceramic powder is at least one of Al.sub.2 O.sub.3, ZrO.sub.2, ZrSiO.sub.4, ZrTiO.sub.4, TiO.sub.2, RO--TiO.sub.2 ceramic such as BaTi.sub.4 O.sub.9, Ba.sub.2 Ti.sub.9 O.sub.20, CaTiO.sub.3 and SrTiO.sub.3, Ln.sub.2 O.sub.3 --TiO.sub.2 ceramic such as Nd.sub.4 Ti.sub.9 O.sub.24 and La.sub.4 Ti.sub.9 O.sub.24, and RO--Ln.sub.2 O.sub.3 --TiO.sub.2 ceramic such as BaNd.sub.2 Ti.sub.5 O.sub.14 and SrPr.sub.2 Ti.sub.3 O.sub.10.
However, the sintered body shows a relatively high dielectric loss, 12.times.10.sup.-4 to 22.times.10.sup.-4, and the ceramic body is blackened to have a low lightness.